Aggression? out of the blue

We see that our puppy (now 3 months old) sometimes suddenly is behaving aggressively.
What i mean, we might walk with him and suddenly might start pulling the leash, trying to escape, jumping like crazy , barking, biting the leash and generally is out of control.
I can't really find up to now what might triggering that , but i have seen most of the times he is "excited" as a man.
This is happening more and more often, was for instance once every 3-4 days, but now i see that in daily basis around 1-2 times.
I have managed to control him (for now at least) by asking him to sit , lay down and wait for a minute , but i am bit worried about this behavior, especially when he will be a big dog.

Other than that is extremely sweet and likes to spend time with us, OK he is biting our legs but i think this is clear puppy behavior...

We see that our puppy (now 3 months old) sometimes suddenly is behaving aggressively.
What i mean, we might walk with him and suddenly might start pulling the leash, trying to escape, jumping like crazy , barking, biting the leash and generally is out of control.
I can't really find up to now what might triggering that , but i have seen most of the times he is "excited" as a man.
This is happening more and more often, was for instance once every 3-4 days, but now i see that in daily basis around 1-2 times.
I have managed to control him (for now at least) by asking him to sit , lay down and wait for a minute , but i am bit worried about this behavior, especially when he will be a big dog.

Other than that is extremely sweet and likes to spend time with us, OK he is biting our legs but i think this is clear puppy behavior...

That's not aggression! Not at all. That is fear!!! Our Willow was so incredibly timid and fearful that she did the same in response to a leash touching her backside, or especially if her collar was grabbed and any pressure applied to move her. She would FLIP out, and would bite to DEFEND herself. I have a couple bloody pics of my hand to prove it when she was about 5 months old, out ice fishing, and she was wandering - I tried to grab her collar to get a leash on her, and she came unglued with FEAR. Not at all aggressive, except to defend her own life.

Now at 2 years old, she has outgrown it for the most part, but she is still fear-filled if her collar is grabbed by hand and pulled. Does great on a leash now.

You just have to be gentle and understand how some in this timid breed react. Slowly introduce them to collar pressure/pulls with gentle sweet talk and treats. Show them, gently, that its not something that is going to hurt them. With our 4 week old puppies we're raising right now, I make sure to pull and tug just a little bit on their collars so they get used to it as "play", and won't fear it so much as they grow (hopefully).